1. Field of the Invention
The present invention relates to fan electronic apparatus which incorporates a plurality of heat-generating semiconductor devices and, more particularly, to an improved electronic device for cooling heat-generating semiconductor devices by a heat sink through the supply of a cooling medium such as air.
The present invention also is concerned with a cooling system which can effecting cool densely packed semiconductor devices.
2. Description of the Related Arts
Electronic apparatuses have been proposed which incorporate a plurality of heat-generating semiconductor devices mounted on a circuit board such as a printed circuit board or a ceramics board. A typical conventional system for cooling semiconductor devices employs cooling fins provided on each such semiconductor device, with cooling air supplied from one to the other side of the electronic apparatus so as to successively cool the heat-generating semiconductor devices. This conventional cooling system, however, cannot cope with the current trend for increasing rates of heat generation by the semiconductor devices in electronic apparatuses of the kind described. Namely, the cooling air after cooling upstream semiconductor devices is heated to a temperature which is too high to efficiently cool downstream semiconductor devices. Under this circumstance, in, for example, Japanese Unexamined Patent Publication 2-34993, a cooling system is proposed in which cooling fins having large heat-radiating areas and, hence, excellent cooling performance are provided on each of the heat-generating semiconductor devices and cooling air is uniformly and separately supplied, without substantial leakage of the air, to the cooling fins of the heat-generating semiconductor devices by a chamber and nozzles disposed above these fins.
Referring to FIG. 31, an electronic apparatus includes a circuit board 1 and a plurality of heat-generating LSIs 2 mounted on the circuit board 1. A heat sink 3 employing cooling fins is provided on each LSI 2, and cooling air is supplied from a chamber 4 onto the LSIs 2 through respective nozzles 5 thereby cooling the LSIs 2. After cooling the heat sink, the air is relieved into an air discharge space 8 formed between adjacent heat sinks 3 through an opening 6, and is discharged through the discharge space 8 in the direction of the arrow 7.
As shown in FIG. 32, the LSIs 2 with their heat sinks 3 are arranged in the form of a regular matrix having plural rows and columns so that air portions 9 from the heat sinks 3 of adjacent columns merge and are discharged in the same direction as indicated by an arrow 10.
Thus, in this known cooling system, each space between adjacent columns of the LSIs 2 or heat sinks 3 form an air discharge passage through which the air fractions, after cooling the semiconductor devices of the adjacent columns, are discharged to the exterior.
The current trend towards higher operation speed and higher packaging density of semiconductor devices of electronic apparatuses requires that the heat-generating semiconductor devices are arranged with a high degree of density, making it difficult to preserve ample space for discharging air between adjacent heat-generating semiconductor devices. Consequently, various problems are encountered.
More particularly, it is difficult to form an air discharge space large enough to receive and discharge air between the adjacent semiconductor devices or heat sinks. Consequently, if the cooling air is supplied to each heat sink at the required rate, the cooling air fractions from these heat sinks successively rush into the common discharge passage of the limited volume, so as to merge and form a high velocity air flow. This results in serious increase in the flow resistance encountering the flow of the air to be discharged. The increased flow velocity also may lead to higher level of fluid noise. Moreover, if the power for forcibly supplying the cooling air is limited, the rate of supply of the cooling air is reduced, thus impairing cooling performance.
The increase of the flow resistance in the air discharge passage also poses a problem that the cooling air cannot be uniformly supplied to all the heat-generating semiconductor devices, because the heat sinks which are closer to the outlet of the cooling passage can receive air at sufficiently large rates, whereas the heat sinks remote from the outlet of the cooling passage cannot be supplied with the cooling air at sufficiently large rates. Such an uneven distribution of air supply rates causes a non-uniform temperature distribution over the heat-generating semiconductor devices of the electronic device.
The above-described problems are experienced not only when the ample space large enough to form air discharge passage cannot be formed between adjacent heat-generating semiconductor devices due to too high density of mounting of these devices, but also when the heat-generating semiconductor devices generate heat at extremely large rates.
In the known electronic apparatuses, the cross-sectional area of the air discharge passage varies according to the space between the heat-generating semiconductor devices or heat sinks. Thus, the cross-sectional area is, to some extent, ruled by the arrangement of the heat-generating semiconductor devices. That is, the direction of the principal flow of the air after the cooling is materially determined by the position of the discharge opening formed in the housing of the electronic apparatus and the arrangement of the heat-generating semiconductors. In other words, it has been difficult to govern the direction of the principal flow of the air after the cooling by the structure of the heat sinks or by ducts and nozzles.
Furthermore, the illustrated known cooling system, in which the spaces formed between adjacent heat-generating semiconductor or heat sinks serve as the air passage for the air portions after cooling merge in each other, suffers from a disadvantage in that the warmed or heated air from upstream heat-generating semiconductors impinge upon the downstream heat-generating semiconductors so as to impair the cooling effect on the downstream semiconductors. Furthermore, the air warmed or heated as a result of cooling of the heat sinks tend to be directed to other portions in the electronic apparatus so as to be resuctioned by the cooling air blower or to heat other electronic components.
Nowadays, higher density of LSI packaging is required also in the field of computers, in order to meet the current demand for higher operation speed of the computers. Consequently, the density of heat generation also is increased partly because each LSI generates greater heat and partly because the density of packaging of LSIs is large. This means that the cooling of LSIs with high efficiency is becoming more important. Previously, as stated before, a conventional system for cooling semiconductor devices mounted on a circuit board such as a printed circuit board or a ceramics board employs cooling fins provided on each such semiconductor device, with cooling air supplied from one to the other side of the electronic apparatus so as to successively cool the heat-generating semiconductor devices. This conventional cooling system, however, cannot cope with the current trend for increasing rates of heat generation by the semiconductor devices in electronic apparatuses of the kind described. Namely, the cooling air after cooling upstream semiconductor devices, is heated to a temperature which is too high to efficiently cool downstream semiconductor devices. In order to obviate these problems, improved cooling systems have been proposed in, for example, Japanese Patent Unexamined Publication No. 2-34993 and Japanese Utility Model Unexamined Publication No. 1-113355 in which cooling fins having large heat-radiating areas and, hence, excellent cooling performance are provided on each of the heat-generating semiconductor devices and cooling air is uniformly and separately supplied from a blower, without substantial leakage of the air, to the cooling fins of these heat-generating semiconductor devices by means of a chamber and nozzles disposed above these fins.
FIGS. 33A and 33B show an example of such improved cooling systems, in particular a system of the type disclosed in Japanese Patent Unexamined Publication No. 2-34993, wherein, a plurality of LSIs 101, representing heat sources, are mounted on a board (not shown). A heat sink 103 composed of fins 102 is provided on each of the LSIs 101. Cooling air is supplied to each LSI 101 through a nozzle which covers the entire area of the heat sink 103 on each LSI 101 so as to cool the latter. Air, after cooling, is discharged from the heat sink 103 through openings 104. Although the cooling air is supplied and distributed uniformly over all the fins in the heat sink 103 through the nozzle which covers the entire area of the heat sink 103, a flow velocity distribution pattern is formed in the heat sink 103 such that the flow velocity is lowest in the region around the bases of the central fins, due to flow resistance, as the air flows through the gaps 102 between the fins towards the openings 104. Consequently, the chip of the LSI exhibits such a temperature distribution that the temperature is highest in the central portion of the chip. It is therefore difficult to uniformly cool each LSI chip.
In view of the above-described problem, Japanese Utility Model Unexamined Publication No. 1-113355 proposes a different air cooling system in which a cooling air entrance 105 on the upper side of the heat sink 103 is restricted so as to cover only the central region of the heat sink 103 thereby concentrating the cooling air to the central fins in the heat sink 103. In this cooling system, although the flow of cooling air is concentrated to the central region of the heat sink, a velocity gradient of the cooling air is established in the heat sink due to flow resistance as the air flows through the gaps between the fins towards openings 106 formed in both side walls of the heat sink 103. It is therefore difficult to remarkably increase the velocity of the cooling air in the region near the base ends of the fins which are disposed in the mid portion of the heat sink. In general, the point at which a fluid collides with a wall is referred to as "stagnation point". A stagnation of the fluid takes place around such a stagnation point. The velocity of fluid can never be enhanced; however, the velocity of the fluid colliding with the wall may be increased. A stagnation point exists in the mid portion of the semiconductor device 101 so as to form a temperature gradient such that the temperature is highest in the mid portion of the semiconductor device, thus failing to uniformly cool the semiconductor device.